In a spread spectrum system, a transmitted signal is spread over a frequency band that is much wider than the bandwidth of the information being transmitted. One technique commonly used in spread spectrum systems is direct sequence (DS) modulation. In DS modulation, each bit of an information-bearing signal is modulated by a higher frequency, binary, pseudorandom code signal. This DS modulation step may simply comprise producing the code signal itself when the information bit is one, and inverting the code signal when the information bit is zero. Each bit of the code signal, and each bit of the DS modulated signal obtained by modulating the information-bearing signal with the code signal, is referred to as a "chip."
Once the DS modulated signal has been produced, it is applied to modulate an intermediate frequency (IF) carrier using any one of several phase or frequency shift keying techniques, e.g., quadrature phase shift keying (QPSK), minimum shift keying (MSK), and bi-phase phase shift keying (BPSK). The IF modulated signal is then up-converted to RF and transmitted. At the receiver, the received signal is first down-converted to an IF signal, and the IF signal is then input to a demodulator that recovers the DS modulated signal. The DS modulated signal is input to a correlator, which "despreads" the DS modulated signal using the same pseudorandom code signal that was used in the transmitter during the DS modulation step. This correlation step permits extraction of the transmitted information-bearing signal, even in the presence of noise or jamming.
Since most DS spread spectrum signals have a very low input signal-to-noise ratio (SNR), the demodulation and correlation are carried out non-coherently. A typical approach is to provide quadrature demodulation of the IF signal, followed by quadrature correlation. In the prior art, a quadrature demodulator is typically followed by a separate and fully independent quadrature correlator circuit.
There are several well known techniques for implementing quadrature demodulators. A common analog quadrature demodulator is configured using mixers and one or more low pass filters. Another approach (usually implemented digitally), is to perform a Hilbert transform to obtain the quadrature component of the signal being demodulated. The advantage of using a Hilbert transform to perform the quadrature demodulator function is that better amplitude and phase match between two channels can be achieved without requiring adjustment of the circuit.
The Hilbert transform can easily be implemented in a finite impulse response (FIR) filter structure. FIR filters have previously been provided using charge-coupled devices (CCDs), for example, as described in U.S. Pat. No. 4,156,858. In this reference, an input signal to the CCD is either a voltage or current that is converted to charge for internal storage; the filter function is implemented by converting the charge information to a voltage or current for output from the device.
Correlators have also been implemented in CCDs. As in the CCD filter discussed above, the input signal to the correlator is a voltage or current that is converted to charge for storage in the device.
In a conventional DS receiver in which both the demodulator and correlator are implemented in separate CCDs, it has in the past been necessary to convert the charge at the output of the CCD demodulator to a voltage, and then reconvert this voltage back to a packet of charge for processing by the CCD correlator. These conversion steps introduce errors and unnecessary complexities into the system. Accordingly, it would be desirable to implement the demodulator and correlator in an integral device that avoids unnecessary conversions between voltage and charge, thereby minimizing distortion and losses.